Light guides suitable for illuminated displays

ABSTRACT

A light guide ( 1 ) comprises a housing ( 3 ) defining a light-guiding optical cavity having first and second opposed major faces ( 5, 6 ), and a light source ( 11 ) arranged to direct light into the cavity from one end, to be guided between the major faces. The first major face ( 5 ) comprises a window through which light can be emitted from the optical cavity to be used, for example, for illuminating a graphic display. To improve the light distribution across that first major face ( 5 ), the second major face ( 6 ) comprises a sheet material ( 23 ) having a specularly-reflecting surface ( 24 ) that faces into the optical cavity and has diffusely-reflecting light-extraction elements ( 27 ) applied thereto in a predetermined configuration for causing light to be emitted from the optical cavity through the said window.

FIELD OF THE INVENTION

[0001] The present invention relates to light guides suitable for use inilluminated displays.

BACKGROUND OF THE INVENTION

[0002] It is already known to use light guides to illuminate panels forgeneral lighting purposes and for display applications (e.g. forilluminating signs and advertisements, and also for illuminating liquidcrystal displays). In one form, often referred to as a light box, thelight guide comprises a hollow box-shaped structure defining an opticalcavity, and in another form it comprises a solid light-guiding plate. Inboth forms, a major surface of the guide can be illuminated by lightdirected into the guide in a direction generally parallel to that majorsurface, for example from at least one elongated light source locatedadjacent an edge of the light guide.

[0003] Light guides in the form of hollow light boxes are described, forexample, in EP-A-0 490 279; 0 377 309; and 0 293 182; and in GB-A-2 310525. In each of those light boxes, a prismatic optical film is employedwith a view to achieving a more even distribution of light over thesurface that is being illuminated. In addition, an Application Bulletinentitled “Thin Light Box” and issued in March 1990 by Minnesota Miningand Manufacturing Company of St. Paul, Minn., USA describes the designand construction of light boxes, using Scotch™ Optical Lighting Film incombination with a shaped sheet of V-5115 Scotch™ Light Extractor Film,for use in illuminating graphic displays.

[0004] In the case of light guides in the form of solid light-guidingplates, it is well known to form light-extraction elements of some typeon the rear major surface of the plate with a view to achieving a moreeven distribution of light over the front surface (i.e. the surface thatis being illuminated). In some cases, printed light-extraction elementsare used and are applied directly to the rear surface of thelight-guiding plate. Arrangements of that type are described, forexample, in U.S. Pat. Nos. 5,736,686; 5,649,754; 5,600,462; 5,377,084;5,363,294; 5,289,351; 5,262,928; 5,667,289; and 3,241,256.

[0005] U.S. Pat. No. 5,618,096 describes light-emitting panels ofvarious types and mentions the possibility of providing light-extractingdeformities on one or both sides of a panel to control the amount oflight emitted from any area of the panel. It is also mentioned that thedeformities may be printed on a sheet or film which is used to apply thedeformities to the panel member. WO 92/05535 describes an illuminateddisplay system comprising a transparent panel with a dot matrix appliedto both sides and an opaque backing sheet attached to the rear side. Animage to be illuminated is printed on a further sheet positioned on thefront side of the panel.

[0006] As recognised in previous disclosures, the problems to beaddressed in constructing a light guide for illumination purposesinclude achieving a uniform level of brightness across the panel that isbeing illuminated, and minimising the amount of power required toproduce a level of illumination that is adequate having regard to thecircumstances. As regards the first of those problems, it is often thecase that the panel is more brightly illuminated in the area closest tothe light source, which detracts from the overall visual appearance andeffectiveness of the illumination. As regards the second of thoseproblems, it is clearly highly desirable, from an environmental and acost point of view, that the amount of power used for illuminationpurposes should be kept as low as possible. Moreover, when the power isderived from a battery (as may be the case when LCD and computerdisplays are being illuminated) it is also generally desirable that theamount of power utilized should be minimized so that the battery can bekept as small and light as possible.

[0007] In addition to those considerations, it would be advantageous tobe able to produce, comparatively easily and in a cost-effective manner,light guides of widely-differing dimensions that would be suitable foruse in a variety of situations but would, nevertheless, function with ahigh level of efficiency.

SUMMARY OF THE INVENTION

[0008] The present invention provides a light guide comprising a housingdefining a light-guiding optical cavity having first and second opposedmajor faces, and at least one light source arranged to direct light intothe cavity from one end, to be guided between the major faces; whereinthe first major face comprises a window through which light can beemitted from the optical cavity, and the second major face comprises asheet material having a specularly-reflecting surface that faces intothe cavity and has diffusely-reflecting light-extraction elementsapplied thereto in a predetermined configuration for causing light to beemitted from the optical cavity through the said window.

[0009] The term “light extraction element” in this context indicates astructure capable of reflecting light at such an angle that it will beemitted from the optical cavity through the said window. In a preferredform, the light extraction elements are printed elements formed in adiffusely-reflecting material. As used herein, the term “printed”includes the case in which the diffusely-reflecting material isdeposited by a conventional printing process involving temporary contactbetween a printing surface (in which the shape of at least one lightextraction element is pre-defined) and the surface on which the lightextraction elements are to be formed. It also includes the case in whichthe diffusely-reflecting material is deposited by being projected atpredetermined locations onto the surface on which the light extractionelements are to be formed.

[0010] Light guides in accordance with the invention can be produced indifferent sizes using comparatively lower cost materials and in a mannerthat is appropriate to high volume production, and can enable theeffective, uniform, and efficient illumination of display panels usingavailable light sources.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] By way of example, embodiments of the invention will be describedwith reference to the accompanying drawings, in which:

[0012]FIG. 1 is a perspective view of a light guide in accordance withthe invention;

[0013]FIG. 2 is a diagrammatic perspective view of a light guide,similar to that shown in FIG. 1, the light guide being shown partlyexploded;

[0014]FIG. 3 is a diagrammatic cross-sectional view of the light guidein exploded form on the line III-III of FIG. 2;

[0015]FIG. 4 illustrates the rear face of a light guide of the typeshown in FIGS. 1 to 3;

[0016]FIG. 5 is a graph illustrating a characteristic of the rear faceof a light guide of the type shown in FIGS. 1 to 3;

[0017]FIGS. 6 and 7 are views, similar to FIGS. 3 and 4, of anotherlight guide and of the rear face of that light guide;

[0018]FIG. 8 is a graph, similar to FIG. 5, for the rear face shown inFIG. 7; and

[0019]FIG. 9 illustrates a modification of the light guide of FIGS. 2and 3.

DETAILED DESCRIPTION

[0020] The light guide 1 shown in FIG. 1 comprises a box-like housing 3defining an optical cavity. The housing 3 has opposed major faces 5, 6,and opposed narrow sides 7, 8 and 9, 10. An elongate light source 11 isarranged adjacent one of the narrow sides 7 to direct light into theoptical cavity in a direction generally parallel to the planes of themajor faces 5, 6. One of the major faces (the face 5) forms a windowthrough which light can be emitted from within the optical cavity andused for illumination purposes.

[0021] The optical cavity 13 inside the housing 3 is visible in thediagrammatic illustration of FIG. 3. The narrow side 7 of the housingadjacent the light source 11 comprises an optical sheet material 15forming a window through which light from the source 11 can enter thelight guide 1. Preferably, the sheet material 15 has a structuredsurface on the side remote from the light source, to redirect the lightfrom the source 11 and ensure that the light that passes through thiswindow enters the optical cavity 13 preferentially in a directiongenerally parallel to the planes of the faces 5, 6. The optical sheetmaterial 15 may, for example, have a structured surface comprising aseries of ridges and grooves formed by a plurality of paralleltriangular prisms. A similar use of sheet material of that type isdescribed in EP-A-0 293 182. In the light guide 1, the material 15 ispreferably oriented so that the prisms extend parallel to the elongatelight source. Suitable sheet material is available, under the tradedesignation “Scotch™ Optical Lighting Film” from Minnesota Mining andManufacturing Company of St. Paul, Minn., USA.

[0022] The narrow side 8 of the light guide 1 opposite the window 15 hasa reflecting surface 17 on the side facing into the optical cavity 13.This reflecting surface, which is preferably a highly-efficientspecularly-reflecting surface, can be provided by any suitable materialbut is preferably provided by a multi-layer optical film of the typedescribed in U.S. Pat. No. 5,882,774 and WO97/01774. A suitablealternative material is a silver reflective film, for example the filmavailable under the trade designation “Silverlux”, from Minnesota Miningand Manufacturing Company of St. Paul, Minn., USA.

[0023] The other two opposed narrow sides 9, 10 of the light guide alsohave reflecting surfaces 18 facing into the cavity. In this case, thereflecting surfaces are preferably provided by a film materialavailable, under the trade designation “Light Enhancement Film” fromMinnesota Mining and Manufacturing Company of St. Paul, Minn., USA.However, any other suitable reflecting material can be used: generally,it has been found that a diffusely-reflecting material is preferablewhen the length/width ratio of these narrow sides is less than 10 andthat a specularly-reflecting material is preferable when this ratio isgreater than 10. It will be appreciated that this ratio corresponds tothe length/thickness ratio of the light guide 1 (otherwise known as its“aspect ratio”).

[0024] The front face, or window, 5 of the light guide comprises anoptical sheet material 19 that, preferentially, guides the light fromthe source 11 along the optical cavity 13 between the faces 5, 6 andpermits light to leave the optical cavity only when it is incident onthe material 19 at certain angles. More specifically, the sheet material19 has a smooth surface facing into the optical cavity and, on the sidefacing away from the optical cavity, a structured surface comprising aseries of ridges and grooves formed by a plurality of paralleltriangular prisms whereby light incident on the material 19 whiletravelling along the optical cavity 13 will be totally internallyreflected provided it is incident on the material 19 within apredetermined angular range. As such, the material 19 may be the same asthe material 15 and, in this case, the material is oriented so that theprisms extend in a direction at right angles to the direction of extentof the light source 11 as indicated in FIG. 2. A similar use of materialof that type is described in EP-A-0 293 182. To protect the prismaticstructures on the sheet material 19, a further panel 21 may bepositioned adjacent the material 19 on the outside of the light guidehousing. This further panel is not essential but, when provided, it maycomprise a sheet of opalescent light-diffusing material to enhance evenfurther the uniformity of the light that passes through the sheetmaterial 19.

[0025] The rear face 6 of the light guide 1 comprises a sheet material23 which provides a highly-efficient specularly-reflecting surface 24facing into the optical cavity 13. The reflecting surface 24 should besuch that its reflectivity is not reduced substantially for light thatis incident in directions other than normal to the surface, and is atleast 90% (preferably at least 98%). Preferably, the sheet material 23is a multi-layer optical film of the type described in U.S. Pat. No.5,882,774 and WO 97/01774. A suitable alternative material, particularlyfor light guides that have a comparatively low aspect ratio (less thanabout 10), is available, under the trade designation “Silverlux”, fromMinnesota Mining and Manufacturing Company of St. Paul, Minn., USA.

[0026] As described in greater detail below, the specularly-reflectingsurface 24 carries diffusely-reflecting light-extraction elements in apredetermined configuration to cause light to be emitted from theoptical cavity 13, through the window 5, in a controlled manner. In thiscase, the light-extraction elements comprise an array of dots 27 formedin a diffusely-reflecting material on the surface 24 as shown in FIG. 4.

[0027] In FIGS. 2 and 3, the light source II is shown as being locatedin a three-sided housing 25, the open side of which is positionedadjacent the sheet material 15 forming the entry window of the lightguide 1. The housing 25 is constructed with a view to ensuring that thelight source 11 directs as much light as possible into the opticalcavity 13 and, to that end, the internal surfaces of the housing may becovered with a suitable highly-efficient, diffusely reflecting material,for example a reflective paint or sheet material. Alternatively, thelight source 11 could be provided with a parabolic reflector to directthe light from the source towards the optical cavity 13, or it could bereplaced by a suitable apertured light source. The use of the sheetmaterial 15 in the narrow side 7 of the light guide housing adjacent thelight source 11, although preferred, is not essential.

[0028] The light guide 1 as described above functions as follows. Lightfrom the source 11 (possibly following reflection or redirection at thewalls of the housing 25) enters the optical cavity 13 through the windowmaterial 15 and travels preferentially in a direction parallel to themajor surfaces 5, 6 of the light guide towards the surface 17 where itwill be reflected and returned. However, any light that is incident onthe extraction elements on the rear surface 24 (i.e. the dots 27) willbe diffusely reflected and some of that light will, as a consequence,impinge on the front face 5 of the light guide at such an angle that itcan pass through the optical sheet material 19 and emerge from the lightguide.

[0029] The use of light-extraction elements of various forms to causelight to be emitted from light guides is already well known. In thelight guide of FIGS. 1 to 3, the light-extraction elements 27 (asalready mentioned) comprise an array of dots formed in adiffusely-reflecting material on the specularly-reflecting surface 24.The circular form of the light extraction elements is not essential,however, and they may be of any shape (for example squares, triangles,lines, etc) that can readily be formed by a printing process, and mayeven comprise a mixture of shapes and/or sizes. Light-extractionelements in the form of continuous lines on the reflecting surface 24are also possible. Preferably, the light-extraction elements 27 areformed by being printed directly onto the reflecting surface 24 but theycould, as an alternative, be printed on a transparent sheet which isthen adhered to the surface 24. Moreover, although the use of printedlight extraction elements is preferred, other forms could be employed asdescribed below.

[0030] The printed light extraction elements 27 on the reflectingsurface 24 of the light guide 1 are positioned to provide a requiredillumination pattern over the front face 5 of the light guide. In manycases, a uniform illumination of the face 5 is required and that can beachieved if the percentage area of the surface 24 that is covered by thediffusely-reflecting elements 27 varies (most typically, increases) withthe distance from the light source 11 (measured in the direction atright angles to the direction of extent of the light source). That isillustrated diagrammatically in FIG. 4, in which it will be seen thatthe proportion of the surface 24 of the sheet material 23 that iscovered by the light extraction elements 27 is zero in the regionimmediately adjacent the light source 11 and then increases as thedistance from the light source increases. In FIG. 4, the surfacecoverage of the light extraction elements 27 is shown reaching a maximumvalue at a short distance from the other end of the sheet 23 and thendecreasing slightly in the region furthest from the light source 11.This decrease is provided to accommodate the effects of the reflectivesurface 17 at the far end of the optical cavity 13; the need for it (andits extent), will be determined in each case by the particularconfiguration of the light guide. It should be understood that FIG. 4 ispurely diagrammatic and that the variation in the surface-coverage ofthe light-extraction elements 27 would typically be continuous ratherthan discontinuous as shown in this drawing. A more typical variation inthe surface-coverage of the light-extraction elements 27 is illustratedin FIG. 5 which shows that the surface coverage is zero over the first10% of the length of the optical cavity 13 measured from the lightsource 11 and then increases linearly, reaching 100% (i.e. totalcoverage) at a distance of about 90% of the length of the optical cavitymeasured from the light source. The surface coverage then decreasesslightly at the end of the optical cavity 13 remote from the lightsource 11.

[0031]FIGS. 6 and 7 illustrate a light guide 31 that is generallysimilar to the guide illustrated in FIGS. 2 to 4 but utilises anadditional light source 11′ positioned opposite to the light source 11(i.e. adjacent the narrow side 8 of the housing 3). To enable light fromthe source 11′ to enter the optical cavity 13, the side 8 of the housing3 comprises an optical sheet material 15′ forming a window, rather thanthe reflecting material 17 of FIG. 3. In addition, the rear surface 24of the light guide is provided with a suitably-modified configuration oflight-extraction dots 27′, described in greater detail below.

[0032] The light source 11′ is located in a three-sided housing 25′similar to that of the light source 11 but, like the light source 11, itcould alternatively be provided with a parabolic reflector to directlight from the source into the optical cavity, or be replaced by asuitable apertured light source. The material 15′ forming the windowfrom the housing 25′ into the optical cavity 13 is preferably the sameas the optical sheet material 15.

[0033] The light guide 31 functions in a similar manner to the guide 1described above except that, in this case, light from both sources 11,11′ (possibly following reflection or redirection at the walls of theassociated housing 25, 25′) enters the optical cavity 13 through theassociated window material 15, 15′ and travels preferentially in adirection parallel to the major surfaces 5,6 of the light guide towardsthe light housing at the other end of the optical cavity where some ofthe light will be reflected and returned. Any light that is incident onthe extraction elements on the rear surface 24 (i.e. the dots 27′) willbe diffusely reflected and some of that light will, as a consequence,impinge on the front face 5 of the light guide at such an angle that itcan pass through the optical sheet material 19 and emerge from the lightguide.

[0034] As with the light guide 1 of FIGS. 2 to 4, the printed lightextraction dots 27′ on the reflecting surface 24 of the light guide 31are positioned to provide a required illumination pattern over the frontface 5 of the light guide. In many cases, a uniform illumination of theface 5 is required and that can be achieved if the percentage area ofthe surface 24 that is covered by the diffusely-reflecting dots 27′varies (most typically, increases) with the distance from each of thelight sources 11, 11′ (measured in the direction at right angles to thedirection of extent of the light sources) up to a maximum in the centralregion equidistant from both light sources. That is illustrateddiagrammatically in FIG. 7, in which it will be seen that the proportionof the surface 24 of the sheet material 23 that is covered by the lightextraction elements 27′ is zero in the regions immediately adjacent thelight sources 11, 11′ and then increases in each case as the distancefrom the respective light source increases, reaching a maximum value inthe central region 33 of the surface. It should be understood that FIG.7 is purely diagrammatic and that the variation in the surface-coverageof the light-extraction dots 27′ would typically be continuous ratherthan discontinuous as shown in this drawing. A more typical variation inthe surface-coverage of the light-extraction elements 27′ is illustratedin FIG. 8 which shows that the surface coverage is zero over the first10% of the length of the optical cavity 13 measured from each of thelight sources 11, 11′ and then increases linearly, reaching 100% (i.e.total coverage) in a central region at a distance approaching 40% of thelength of the optical cavity measured from each light source.

[0035]FIGS. 4 and 7 both indicate that the surface-coverage provided bythe light-extraction elements 27, 27′ is varied by changing the numberof dots per unit area of the surface 24 while maintaining a uniform dotsize. As an alternative, the size of the extraction elements can bechanged while maintaining a constant number of extraction elements perunit area of the surface 24 and, as a further alternative, both the sizeof the extraction elements and the number per unit area can be varied.

[0036] In some cases, it may also be appropriate to vary thesurface-coverage of the light extraction elements 27, 27′ transverselyof the optical cavity (i.e. in a direction parallel to the direction ofextent of the light source(s) 11, 11′).

[0037] The extraction elements 27, 27′ of FIGS. 4 and 7 can be printedusing any suitable printing medium that will function as a diffusereflector and is compatible with the reflecting surface 24 and with theprinting process employed. One suitable medium is a highly-efficientdiffusely-reflecting matt white ink. Any suitable printing process canbe used to deposit the printing medium on the surface of the sheetmaterial, including screen printing, gravure printing, offset printing.Ink jet printing may also be employed. In a preferred process, theprinting medium is deposited using a conventional silk screen printingprocess because that is a versatile, comparatively low-cost processwhich is suitable for long production runs but nevertheless enables goodcontrol to be maintained over the size of the extraction elements 27,27′. The extent to which the reflecting surface 24 should be covered bythe printing medium can be determined empirically for a particular lightguide by providing an arbitrary, linearly-varying, pattern of extractionelements 27, 27′ on the surface 24 and comparing the resultingillumination of the front face 5 of the light guide with that achievedin the absence of any extraction elements. The pattern is then adjusted,on the basis of that comparison to yield the illumination required.

[0038] Although it is generally required to achieve a constant level ofillumination across the front face 5 of the light guide 1, 31 there maybe occasions when it is desirable to provide a level of illuminationthat varies across the face 5 in a predetermined manner. For example,the level of illumination across the front face 5 could be matched tothe image that is being illuminated so that the brighter parts of theimage receive more light and the darker parts of the image receive less.That could be achieved, for example, by first providing, on thereflecting surface 24, the pattern of extraction elements 27, 27′required to provide uniform illumination of the front face 5 andsubsequently superimposing, on that pattern, further extraction elementsarranged in an image-dependent configuration. The further extractionelements could, for example, be printed directly over the elements 27,27′ or they could be printed on a separate transparent sheet which isthen adhered to the already-printed surface 24. As an alternative, thefurther extraction elements could be provided on the smooth surface ofthe optical sheet material 19 in the front face 5 of the light guide, inwhich case they should be formed of a translucent material, capable ofboth reflecting and transmitting incident light from within the opticalcavity 13. As a further alternative, the extraction elements 27, 27′could be omitted from the reflecting surface 24 (at least in certainareas) with only the further, image-related, extraction elements beingprovided.

[0039] An arrangement of the type referred to in the previous paragraphis illustrated diagrammatically in FIG. 9, for a light guide intended tobe illuminated by a single light source 11 as in FIGS. 2 and 3. Thereflecting surface 24 carrying the pattern of extraction elements 27required to provide uniform illumination of the front face of the lightguide is shown, in combination with a sheet 35 carrying the graphicimage to be illuminated. Superimposed on the reflecting surface 24 is atransparent sheet 37 carrying further extraction elements 39 arranged inan image-related configuration whereby the brighter parts of the imagereceive more light and the darker parts of the image receive less.Typically, the graphic image on the sheet 35 is a digitally-printedimage, in which case the image data file that is used to print the imagecan also be used to print the extraction elements 39 on the sheet 37taking account, if required, of factors such as the nature of the inksused in the graphic image and the spectral sensitivity of the human eye.

[0040] Although FIG. 9 shows the further extraction elements 39 locatedon the separate sheet 37, it will be understood that they couldalternatively be located on the reflecting surface 24. In that case,when using a digital printing process, all of the required lightextraction elements (i.e. elements 27 as well as elements 39) canreadily be deposited on the surface 24 together.

[0041] The use of a sheet material 23 for the rear face of the opticalcavity 13 of the light guides 1, 31 is advantageous because such amaterial is easy to handle, not only during the actual printing process(when the fact that the sheet material has flat, unstructured, surfacesis a particular advantage) but also during any subsequent drying andstorage stages prior to assembly of the light guide. When in use in thelight guide, the reflective sheet material 23 prevents light fromleaving the optical cavity 13 through the rear face 6 and thus enhancesthe illumination of the front face 5. In addition, any scratches on thesurface of the reflective sheet material (which might arise, forexample, during handling or assembly of the light guide) will notadversely affect the illumination of the front face 5. Moreover, becausethe extraction elements 27 are printed on a planar reflecting surface 24there is also no risk of more interference patterns arising even whenthe extraction elements are disposed in a regular array.

[0042] The printing medium used to form the extraction elements 27, 27′is selected for compatibility with the sheet material to which it isapplied, as well as for its durability and diffusely-reflectingcharacteristics. Highly-opaque white inks are preferred and it has beenfound that the best printing quality is obtained using UV-cured inks ina screen printing process. Screen printing offers the further advantagethat the ink layer deposited on the sheet material 23 is thick and,consequently, comparatively robust and also less likely to fade anddiscolour. Moreover, unlike some printing processes, the screen printingprocess does not entail the application of high pressures to thematerial 23 and is less likely to damage the latter. It can also be usedto apply the printing medium to a wide range of different sheetmaterials in a wide range of sizes. It should be understood, however,that other media could be employed to form the light extraction elements27, 27′, as could other processes including, for example, laserprinting, ink jet printing, thermal transfer printing and thermal inkjet printing.

[0043] In some cases, the extraction elements 27, 27′ may be formed byother methods, including, for example, surface roughening of the sheetmaterial or deposition of diffusely-reflecting material (which mayinclude particles) in a required configuration using coating or sprayingprocesses.

[0044] A hollow light guide as described above with reference to FIGS. 1to 3 or 6 can be fabricated in such a way that it is comparativelylightweight. That is a particular advantage when the light guide islarge in size (for illuminating large signs, for example), andespecially when it is required to be installed in a less accessiblelocation. It also makes the use of thicker light guides (which offer thepossibility of admitting more light into the optical cavity 13) morepractical. It is also comparatively simple to fabricate light guides ofthis type in many different sizes and, in particular, with widelydiffering aspect ratios (i.e. different length/thickness ratios). Forexample, light guides of this type can be produced with aspect ratios assmall as 5 and as large as 100 and, in both cases, will functionefficiently. Light guides with small aspect ratios offer the advantagethat the light admitted to the optical cavity 13 undergoes fewerreflections before it is emitted through the window 5. Consequently, thelevel of accuracy required in the configuration of the light extractionelements 27, 27′ is lower.

[0045] Of particular interest in the field of illuminated signs is thefact that light guides of the type shown in FIGS. 1 to 3 and 6 can befabricated with widths as small as 10 cm and even, depending on the sizeof the sign, as small as 3 cm. Light guides of this type, having lowaspect ratios (typically less than 10) and using conventionalfluorescent tubes as light sources, have been found to have performanceefficiencies that compare favourably (and, in some cases, veryfavourably) with those that can be achieved using solid light guides. Inthe alternative case in which the light guides have larger aspectratios, they are found to be better able than solid light guides toaccommodate some degree of misalignment of the light source.

[0046] The light sources employed with the light guides 1, 31 are notrequired to have an elongate form as illustrated. Other light sourcescould be employed including, for example, light emitting diodes (LEDs)Depending on the form of the light source, more than one source may beused to direct light into the optical cavity 13 through the adjacentside of the housing 3. In that case, the surface-coverage of the lightextraction elements 27, 27′ may also vary transversely of the opticalcavity (i.e. between the sides 9, 10).

[0047] The light guides illustrated in FIGS. 1 to 3 and 6 have beendescribed above as being used to illuminate a graphic display but theycould be used for other purposes including, for example, to illuminateliquid crystal displays.

[0048] An example of an illuminated sign incorporating a light guide ofthe type illustrated in FIGS. 1 to 3 will now be described.

[0049] The housing 3 of the light guide 1, excluding the front majorface 5, may be a one-piece vacuum-formed construction of any suitablematerial, for example PVC (polyvinylchloride). Alternatively, thehousing may be formed from several pieces of, for example, an acrylicmaterial, each providing one side of the housing, which are securedtogether in any suitable manner. The housing is approximately 60×60×4.5cm.

[0050] The internal surface of the rear major face 6 of the housing iscovered with a sheet 23 of a specularly-reflective, multi-layer, opticalfilm of the type described in U.S. Pat. No. 5,882,774 and WO 97/01774,having a reflectivity in a direction normal to the surface of the filmof at least 98%. The surface 24 of the film 23 facing into the housing 3carries a printed dot pattern providing a percentage surface coveragethat varies as represented in FIG. 5. The dot pattern was screen printedon the surface 24 of the film 23 using a white UV-cured ink of a typeformulated for the printing of compact discs (a suitable ink beingavailable from NOR-COTE of Eastleigh, Hampshire, England). The variationin percentage surface coverage of the surface 24 by the ink was achievedby varying the size of the dots while maintaining a constant number ofdots per unit area of the surface (based on transverse lines of dotscontaining about 20 dots per inch (2.54 cm)).

[0051] The internal surface of one narrow side 7 of the housing 3 iscovered with a sheet 15 of the above-mentioned “Scotch™ Optical LightingFilm”, arranged with the prisms facing into the housing and extendingparallel to the long edges of this side of the housing. The internalsurface of the opposite narrow side 8 of the housing 3 is covered withthe same specularly-reflective film material as the internal surface ofthe rear major face 6 but without the printed dot pattern. The internalsurfaces of the remaining two narrow sides 9, 10 of the housing 3 arecovered with the above-mentioned “Light Enhancement Film”.

[0052] The housing 3 is closed with a sheet 19 of the above-mentioned“Scotch™ Optical Lighting Film”, forming the front major face 5. Thefilm is arranged so that the prisms are on the outside of the housingand extend between the narrow sides 7 and 8.

[0053] The light guide module thus formed was put into a sign housingand provided with a 60 cm long, 14W fluorescent lighting tube located,within a high-reflectance housing 25, adjacent the narrow side 7 of thelight guide housing 3 and arranged to direct light into the latter. Itwas found that the front major face 5 of the housing 3 was illuminatedwith a high degree of uniformity and to a level sufficient to provideeffective illumination of a graphic image located in front of the face5.

1. A light guide comprising a housing defining a light-guiding opticalcavity having first and second opposed major faces, and at least onelight source arranged to direct light into the cavity from one end, tobe guided between the major faces; wherein the first major facecomprises a window through which light can be emitted from the opticalcavity, and the second major face comprises a sheet material having aspecularly-reflecting surface that faces into the cavity and hasdiffusely-reflecting light-extraction elements applied thereto in apredetermined configuration for causing light to be emitted from theoptical cavity through the said window.
 2. A light guide as claimed inclaim 1, in which the specularly-reflective surface has a reflectivityof at least 90% for light incident on the surface at any angle.
 3. Alight guide as claimed in claim 1 or claim 2, in which the sheetmaterial having the specularly-reflecting surface is laminated to aninternal surface of the housing
 4. A light guide as claimed in any oneof the preceding claims, in which the light-extraction elements areprinted elements formed in a diffusely-reflecting material.
 5. A lightguide as claimed in claim 4, in which the light-extraction elements areprinted directly on the specularly-reflecting surface.
 6. A light guideas claimed in any one of the preceding claims, in which the percentagearea of the specularly-reflecting surface that is covered by lightextracting-elements is not constant over the whole area second majorface.
 7. A light guide as claimed in claim 6, in which, in any region ofthe second major surface, the percentage area of thespecularly-reflecting surface that is covered by lightextracting-elements varies with the distance of that region from thelight source.
 8. A light guide as claimed in any one of the precedingclaims, including a second light source arranged to direct light intothe cavity from the end opposite the first-mentioned light source, to beguided between the major faces.
 9. A light guide as claimed in any oneof the preceding claims, in which the first major face has a structuredsurface comprising a plurality of parallel prisms on the side remotefrom the optical cavity.
 10. A light guide as claimed in any one of thepreceding claims, in which a display that is to be illuminated ispositioned in front of the window.
 11. A light guide as claimed in claim10, in which at least some of the diffusely-reflecting light-extractionelements are applied to the specularly-reflecting surface in aconfiguration that is related to the display.
 12. A light guide asclaimed in claim 10, including diffusely-reflecting light-extractionelements applied to the window, on the side facing into the opticalcavity, in a configuration that is related to the display.
 13. A lightguide substantially as described herein with reference to, and asillustrated by, FIGS. 1 to 5; or FIGS. 6 to 8; or FIGS. 1 to 5, asmodified by FIG. 9; or FIGS. 6 to 8 as modified by FIG. 9, of theaccompanying drawings.